In the recent “Horrors of Homogenization” post I panned the warming adjustments applied by NCDC to the Paraguayan raw surface air temperature records, which are similar to those applied by GISS and BEST. However, further study suggests that these adjustments may – repeat may – not be quite so outrageous as they look, and in the interests of balance and objectivity this post presents a review of the Paraguayan records that addresses the question of whether they really do contain a cooling bias that would justify warming corrections. And the fact that the review fails to reach a conclusion serves as an example of how difficult it can sometimes be to make sense of conflicting temperature records in the same area.
We begin with a continental perspective. Figure 1 plots temperature trends since about 1950 (record length varies) at 90 selected South American stations. All those blue dots in Paraguay; are they caused by genuine cooling, or are they caused by Paraguay?
Figure 1: Distribution of records showing warming and cooling, South America
A strong statistical case can be made that they are caused by Paraguay. Only 13 of the 80 records outside Paraguay (16.25%) show cooling, and if we apply this probability to Paraguay we find that the chances that almost all of the records there will show cooling are effectively zero. And because Paraguay’s boundaries do not follow any physiographic features that might be expected to impact climate there is no obvious reason why this probability should not be applied. (The only other South American country that shows net cooling since 1950 is Chile, but Chile is separated from the rest of the continent by the Andes.)
But when we zoom in on Paraguay and the surrounding area (Figure 2) we find that the correlation between cooling and Paraguay’s borders is not exact. Of the ten stations in Paraguay that have reasonably continuous records going back at least fifty years two (Puerto Stroessner and Pedro Juan Caballero) show warming, and Camiri and Yacuiba in Bolivia also show net cooling rather than warming:
Figure 2: Distribution of records showing warming and cooling, Paraguay and surrounding area
What does the Paraguayan cooling look like? It’s defined by the eight congruous raw records shown in Figure 3, which include Puerto Stroessner even though it shows overall warming but exclude Encarnacion, which is an erratic record that matches none of the other records in the area (all the raw temperature data used in this post are downloaded from the BEST station list). A trend line drawn through the mean of all the records shows about a degree C of net cooling since 1940, with effectively all of it occurring between the early 1960s and the mid-1970s:
Figure 3: Monthly means of Paraguayan records showing cooling (Asuncion, Puerto Casado, Pilar, San Juan Bautista, Puerto Stroessner, Concepcion, Mariscal, Bahia Negra. Mean of all records shown in black)
And what do the records from Argentina and Brazil that surround Paraguay look like? As shown in Figure 4 they are also reasonably congruous but collectively they show about six-tenths of a degree of gradual warming since 1940 (the records include the Paraguayan city of Pedro Juan Caballero, which connects seamlessly with the Brazilian city of Ponta Pora and is part of the same metro area):
Figure 4: Monthly means of Argentinian & Brazilian records showing warming (Corrientes, Resistencia, Formosa, Posadas, Las Lomitas, Ponta Pora, Pedro Juan Caballero, Corumba. Mean of all records shown in black)
Figure 5 plots the difference between the means of the two sets of records shown in Figures 3 and 4. The peaks and troughs line up but the Paraguay records show a degree C of cooling relative to the Argentina-Brazil records between 1965 and ~1976.
Figure 5: Comparison of means of Paraguay and Argentine/Brazil records shown in Figures 3 and 4
And now we come to the $64,000 question. Is this 10-year cooling period in (most of) Paraguay real or artificial? If it’s artificial it will have one of the following physical causations:
Station moves: BEST, which records these events in its data set, identifies no station moves in Paraguay between 1965 and 1976.
Time-of-observation changes: BEST identifies none of these either.
It is of course possible that there were unrecorded station moves or t_obs changes between 1965 and 1976, but if there were we should see accompanying stair-step shifts in the Paraguayan records that can be identified by subtracting them from the mean of the Argentina/Brazil records. When I applied this approach to the eight records shown in Figure 3, however, I identified only one such shift – in the Asuncion record at the end of 1966 – and it explains only about half of the total ~1.5C cooling differential between Asuncion and the Argentina/Brazil records since 1953:
Figure 6: Asuncion record minus Argentina/Brazil mean (black line is running annual average)
Land use changes: These can cause gradual temperature shifts, but there is no evidence for any land-use changes in Paraguay over the 1965-76 period that were not replicated to at least some extent next door in Argentina and Brazil.
The only event I found that coincides in time with the 1965-1976 period is the tenure of Colonel Gumersindo Adolfo Da Silva as head of Paraguay’s Dirección de Meteorología e Hidrología between 1965 and 1977. Could the colonel have implemented changes over this period that led to nationwide cooling? No way of knowing.
And although the distribution of red dots in Figure 1 makes it look unlikely we also have to consider the possibility that the Paraguayan records are correct and that the Argentinian and Brazilian records are warming-biased– did I hear someone say “urban warming”? I looked into this question and here are the results. First a tabulation of station populations and trend-line warming/cooling gradients since ~1950 in degrees C/century:
And second an XY plot of the Table data:
Figure 7: Warming/cooling trend versus population, all records
The data show no trend until we delete Asuncion, whereupon we see some evidence that urban warming could be having an effect, although the results are not exactly compelling. (One problem is that to evaluate urban warming impacts we need to know where the station was at all times in its history, but in many cases we don’t even know where the station is now. The incredibly precise lat-longs BEST gives for the Asuncion station – 25.34792 degrees south, 57.67917 degrees west – place it in a swamp over the border in Argentina.)
The only thing left to consider is the evidence that suggests that the Paraguayan cooling may be a real climatic effect. For what it’s worth, here it is:
1. One record in Paraguay – Pedro Juan Caballero – shows a warming gradient that matches the warming gradients in the adjacent Argentinian and Brazilian records. Another (Puerto Stroessner) shows overall warming. Two records in Argentina – Corrientes and Resistancia – show little or no warming.
2. Two adjacent records in Bolivia, Camiri and Yacuiba, also show cooling. The trend, however, is different to the cooling trend in Paraguay:
Figure 8: Camiri (blue) and Yacuiba (red) monthly records versus Paraguay mean (black)
3. The large “hump” in the Paraguayan record between 2001 and 2004 is not seen in records outside Paraguay. If this feature is real – and it’s hard to see how it could be artificial – it indicates that things happen to the climate in Paraguay that don’t happen elsewhere. (Similar humps occur in some Amazon Basin records between 1960 and 1970.)
So where does all this leave us? It leaves us unable to say whether the Paraguayan records are cooling biased or not, and therefore unable to prove that the warming adjustments applied to them by BEST, GISS and NCDC are invalid (although if they are valid it’s for the wrong reasons; but that would have to be the subject of a separate post). How do we resolve this uncertainty, if in fact it can be resolved? And how do we treat the Paraguay records if it can’t?
Addendum: Euan has suggested that it might be instructive to post a graph of temperature data from Chile, the only other country in South America which shows net cooling since 1950. The graph below plots the records for (from south to north) Punta Arenas, Isla Huafo, Puerto Montt, Valdivia, Isla Juan Fernandez, La Serena and Copiapo, with the mean again shown in black. Note that Punta Arenas and Copiapo are almost 3,000km apart.
Figure 9: Seven Chilean surface air temperature records